Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 97)

97 Publication 1746-UM614B-EN-P - September 2007
Appendix
C
Thermocouple Descriptions
The descriptions of thermocouples J, K, T, E, R, and S were extracted
from NBS Monograph 125 (IPTS–68) issued March 1974. We also
describe types C and D.
J Type Thermocouple
(iron versus copper-nickel
<Constantan
(1)
>)
The J thermocouple is the least suitable for accurate thermometry
because there are significant nonlinear deviations in the
thermoelectric output from different manufacturers. The total and
specific types of impurities that occur in commercial iron change with
time, location of primary ores, and methods of smelting.
Type J thermocouples are recommended by the ASTM [1970] for use
in the temperature range from 0
… 760 °C (32…1400 °F) in vacuum,
oxidizing, reducing or inert atmospheres. If used for extended times
above 500 °C (932 °F), heavy gauge wires are recommended because
the oxidation rate is rapid at elevated temperatures.
They should not be used in sulfurous atmospheres above 500 °C
(932 °F). Because of potential rusting and embrittlement, they are not
recommended for subzero temperatures. They should not be cycled
above 760 °C (1400 °F) even for a short time if accurate readings
below 760 °C (1400 °F) are desired at a later time.
The negative thermoelement, a copper-nickel alloy, is subject to
substantial composition changes under thermal neutron irradiation,
since copper is converted to nickel and zinc.
Commercial iron undergoes a magnetic transformation near 769 °C
(1416 °F) and <an alpha – gamma> crystal transformation near 910 °C
(1670 °F). Both of these transformations, especially the latter, seriously
affect the thermoelectric properties of iron, and therefore, the Type J
thermocouples. If Type J thermocouples are taken to high
temperatures, especially above 900 °C (1652 °F), they will lose
accuracy of their calibration when they are recycled to lower
temperatures.
(1)
It should be noted that the Constantan element of Type J thermoelements is not interchangeable with the Constantan element of Types T or N due to the different ration of
copper and nickel in each.